Molecular pathology is the examination at a molecular level of the DNA, mRNA, and proteins that cause or are otherwise associated with disease. From this examination important information about patient diagnosis, prognosis, and treatment options can be elucidated. The practice of molecular pathology is generally divided into two main areas: (i) analysis of DNA, mRNA, and proteins in intact cells (in-situ), and (ii) analysis of these biological materials after they have been extracted from tissues. The first category, to which the present invention is primarily directed, has the advantage that it allows the pathologist or scientist to study the histopathologic architecture or morphology of the tissue specimen under the microscope at the same time that the nucleic acid or proteins are being assayed. These techniques include immunohistochemistry (IHC) which looks at proteins, in-situ hybridization (ISH) which looks at nucleic acids, histochemistry (HC) which looks at carbohydrates, and enzyme histochemistry (EHC) which looks at enzyme chemistry. For example, ISH can be used to look for the presence of a genetic abnormality or condition such as amplification of cancer causing genes specifically in cells that, when viewed under a microscope, morphologically appear to be malignant. ISH is also useful in the diagnosis of infectious diseases as it allows detection not only of a microbial sequence but also of precisely which cells are infected. This may have important clinicopathologic implications and is an effective means to rule out the possibility that positive hybridization signal may have come from an adjacent tissue of no clinical concern or from blood or outside contamination.
IHC utilizes antibodies which bind specifically with unique epitopes present only in certain types of diseased cellular tissue. IHC requires a series of treatment steps conducted on a tissue section or cells (e.g. blood or bone marrow) mounted on a glass slide to highlight by selective staining certain morphological indicators of disease states. Typical steps include pretreatment of the tissue section to remove the paraffin and reduce non-specific binding, retrieval of antigens or cell conditioning masked by cross-linking of the proteins from the chemical fixatives, antibody treatment and incubation, enzyme-labeled secondary antibody treatment and incubation, substrate reaction with the enzyme to produce a fluorophore or chromophore highlighting areas of the tissue section having epitopes binding with the antibody, counterstaining, and the like. Most of these steps are separated by multiple rinse steps to remove unreacted residual reagent from the prior step. Incubations can be conducted at elevated temperatures, usually around 37° C., and the tissue must be continuously protected from dehydration. ISH analysis, which relies upon the specific binding affinity of DNA or RNA probes with unique or repetitive nucleotide sequences from the cells of tissue samples or bodily fluids, requires a similar series of process steps with many different reagents and is further complicated by varying temperature requirements.
In view of the large number of repetitive treatment steps needed for both IHC and ISH, automated systems have been introduced to reduce human labor and the costs and error rate associated therewith, and to introduce uniformity. Examples of automated systems that have been successfully employed include the ES®, NexES®, DISCOVERY™, BENCHMARK™ and Gen II® staining Systems available from Ventana Medical Systems (Tucson, Ariz.). These systems employ a microprocessor-controlled system including a revolving carousel supporting radially positioned slides. A stepper motor rotates the slide carousel placing each slide under one of a series of reagent dispensers positioned above the slides. As described in U.S. Pat. No. 6,352,861 B1, bar codes on the slides and reagent dispensers fully automate the computer-controlled actuation of the dispensers onto the slides so that different reagent treatments can be performed for each of the various tissue samples.
Instrumentation such as the Ventana Medical Systems ES®, Gen II® NexES®, BENCHMARK® and DISCOVERY® systems are fundamentally designed to sequentially apply reagents to tissue sections mounted on one by three inch glass microscope slides under controlled environmental conditions. The instrument must perform several basic functions such as reagent application, washing (to remove a previously applied reagent), jet draining (a technique to reduce the residual buffer volume on a slide subsequent to washing), Liquid Coverslip™ application (a light oil application used to contain reagents and prevent evaporation), and other instrument functions.
The Ventana Medical Systems staining instruments mentioned above process slides on a rotating slide carousel. The instrumentation described herein has the slides fixed in a stationary position and rotates the basic processing stations above the fixed slides. The following details of how the slides are processed, the process algorithm, is the same regardless of the physical configuration.
The process of staining tissue on a slide consists of the sequential repetition of the basic instrument functions described above. Essentially a reagent is applied to the tissue then incubated for a specified time at a specific temperature. When the incubation time is completed the reagent is washed off the slide and the next reagent is applied, incubated, and washed off, etc, until all of the reagents have been applied and the staining process is complete.
It is desirable to permit any staining protocol for any of the slides being run, i.e. any combination of reagents and incubation times. In addition, to stain multiple slides as quickly as possible the instrument should process the slides simultaneously. This is feasible given that most of the time slides are just incubating, thus freeing up time to perform the washing, reagent application and other functions on other slides.
One algorithm to accomplish simultaneous staining (sometimes referred to as the “random access” method) is to create a task and time schedule for each slide in the run, then perform each task on each slide when the schedule calls for it. The problem with this method is that incubation times will not be accurate if the instrument is busy performing a task on one slide when it is time to be washing another slide (thereby completing incubation on that slide). The variation in incubation times will be unpredictable since the total number of slides and the slide protocols vary.
Slide processing using a lock-step algorithm insures that all incubation times are accurate and predictable irrespective of the number of slides processed or the variation in slide protocols. While incubation times are assured, the lock step algorithm implies that incubation times must be an increment of the fundamental incubation time period. For example, with an incubation cycle of two minutes, the total incubation times must be multiples of two, i.e., two, four, six, eight etc. minutes in duration. However, the preferred embodiment of the present invention uses a four minute incubation time. Generally this is not a particular limitation since typical incubation times are an order of magnitude longer than the fundamental incubation period.
Prior art staining systems typically include either convection or radiation to warm the samples above laboratory ambient temperatures for steps requiring elevated temperatures. Heating the slide improves staining quality by acceleration of the chemical reaction and can permit a reaction temperature more closely matching body temperature (about 37° C.) at which antibodies are designed to react. While such convection or radiant heating systems have been generally suitable for IHC, which is antibody-based, they are less suitable for ISH, which is nucleic acid-based and requires higher and more precise temperature control in order to denature DNA. In order to denature the DNA double helix of both the target sample and the probe so as to render them single stranded, the temperature must be raised above the melting point of the duplex, usually about 94° C. Precise temperature control is also required in ISH to effect probe hybridization at the desired stringency. The selected temperature must be low enough to enable hybridization between probe and target, but high enough to prevent mismatched hybrids from forming.
Hot air convection, conduction or radiant heat heating units typically employed with prior art automated tissue stainers do not permit the temperature of individual slides to be separately controlled. With prior art systems all of the slides are heated to the same temperature at any given time during the process. For example, U.S. Pat. Nos. 5,645,114 and 6,180,061 to Bogen et al. disclose a dispensing assembly adapted to carry a plurality of microscope slides. Individual slide holders containing resistive heating units are provided. However, with the assembly taught by Bogen et al., all of the slides would be heated to a common temperature because no means are disclosed for separate heating controls or for shielding slides from heat generated by adjacent slides.
Other difficulties frequently encountered in both IHC and ISH testing results from the manner in which the tissues are typically preserved. The mainstay of the diagnostic pathology laboratory has been for many decades the formalin-fixed, paraffin-embedded block of tissue, sectioned and mounted upon glass slides. Fixation in such a preservative causes cross-linking of macromolecules, both amino acids and nucleic acids. These cross-linked components must be removed in the case of ISH, to allow access of the probe to the target nucleic acid, and, in the case of IHC, to allow the antibody to recognize the corresponding antigen. “Unmasking” the antigen and/or nucleic acid is typically accomplished manually with multiple pretreatment, protolytic digestion, and wash steps.
Prior to staining, complete removal of the paraffin is also required so that it does not interfere with antibody or probe binding. Manual deparaffinization normally is achieved by the use of two or three successive clearing reagents that are paraffin solvents such as xylene, xylene substitutes or toluene. However, new automated methods that are largely based on physical separation mechanisms are revealed in U.S. Pat. No. 6,544,798 B1 to Christensen et al., which do not require toxic solvents and are aqueous-based.
The foregoing discussion of the prior art largely derives from Richards et al. U.S. Pat. No. 6,296,809, assigned to Ventana Medical Systems, in which there is described apparatus and methods for automatically staining or treating multiple tissue samples mounted on microscope slides so that each sample can receive an individualized staining or treatment protocol even when such protocols require different temperature parameters. More specifically, there is described in the '809 patent apparatus comprising a computer controlled, bar code driven, staining instrument that automatically applies chemical and biological reagents to tissue or cells mounted or affixed to standard glass microscope slides. According to the '809 patent, a plurality of slides are mounted in a circular array on a carousel which rotates, as directed by the computer, to a dispensing location placing each slide under one of a series of reagent dispensers on a second rotating carousel positioned above the slides. Each slide receives the selected reagents (e.g. DNA probe) and is washed, mixed and/or heated in an optimum sequence and for the required period of time.
According to the '809 patent, individual slides are carried on thermal platforms radially mounted to the carousel. Temperature sensors are also mounted to the slide carousel, individually monitoring and controlling each thermal platform separately. Apparatus made in accordance with the '809 patent is available commercially from Ventana Medical Systems, of Tucson, Ariz. as the DISCOVERY® or BENCHMARK® systems.
The present invention is a modification and improvement over the prior art including the apparatus and methods described in the '809 patent. More particularly, the present invention rather than bringing the slides to the reagent, stain, and wash stations, brings the reagent, stain and wash stations to fixedly positioned slides. That is to say, in the present invention the slides are fixedly positioned in the apparatus, and the various washing, staining and reagent fluids are selectively delivered to the slides. Fixing the slides in position in the apparatus simplifies wiring to the heaters, and also eliminates the potential that a slide may be dislocated by rapid start and stop movement of the slide carousel, which, in a worst case scenario could result in a domino or train-wreck effect where one dislocated slide hits the neighboring slide causing that slide to dislocate, and so forth.